1. Field of the Invention
The present invention relates to explosives. More particularly, the invention relates to a method and apparatus for simultaneously initiating multiple explosive devices for use in various applications, including wellbore applications.
2. Description of Related Art
The process of selectively placing holes in a liner and cement so that oil and gas can flow from a reservoir formation into the wellbore and eventually to the surface is generally known as “perforating.” One such perforation technique includes triggering a detonation system to launch a projectile, such as a shaped charge jet, to perforate and fracture the formation so as to create the flow path.
Typically, a large number of shaped charges are inserted into the wellbore in what is called a gun. The charges are contained in a steel tube, protected from impact and from the well fluids, and are arranged so that they face radially outward from the vertical axis of the carrier. The shaped charge is capable of being initiated by, for example, a detonating chord, which when initiated by a percussion detonator or by an electrical detonator causes the shaped charges to fire and create the hydrocarbon flow path into the wellbore.
The firing of the individual charges can produce interfering shock effects that reduce performance of adjacent shaped charges if initiated simultaneously. Therefore, some separation between charges is required to reduce likelihood that the detonation of an individual charge interferes with the subsequent detonation of an adjacent charge. Typically, the separation distance required for proper firing of charges varies with the particular gun design and depends upon the application. A separation between shaped charges reduces the number of perforations into the formation for a given length gun, which decreases the productivity of the well and therefore increases costs.
Another type of electrically activated detonator, capable of activating explosive devices, such as shaped charges, is the exploding foil initiator (EFI). Conventionally, such a device includes a metallic foil that is connected to a very powerful source of pulsed electric current. A reduced neck section in the foil explosively vaporizes when subjected to a sufficiently high and sufficiently quick current pulse, and that causes a small, thin disk torn from a contiguous insulating material layer to fly a short distance and impact the surface of an explosive, initiating a detonation. Other electrically activated initiators include exploding bridge wire (EBW) initiators, exploding foil “bubble activated” initiators, hot wire blasting caps, etc.
Jitter is the shot-to-shot variation of the time between the electrical pulse and the initiation of a detonation in the main high explosive charge. The lowest value of jitter is determined by the detonator itself, but larger jitter values are always experienced due to the characteristics of the other components in the electrical firing system. Low energy detonators, such as blasting caps, have very slow electro-chemical trains (sequence of electrical and chemical stages) that produce large amounts of shot-to-shot jitter. High-energy detonators, such as EFIs (slappers), utilize large, quick pulses of electricity to minimize the electrochemical train burn times. To fire more than one detonator such that an entire array of detonators function together requires a robust and elaborate electrical distribution system to bring the powerful pulse of electricity to each detonator in the array from a central fast discharge fire set.
Less robust and elaborate conventional electric distribution systems are not satisfactory for simultaneous multiple detonations. The individual firing times of these types of systems can vary by microseconds. A typical detonation wavefront within a secondary explosive travels at a velocity in excess of 6 millimeters/microsecond (RDX is approximately 8 millimeters/microsecond and HNS is more than 6 millimeters/microsecond). Since these secondary explosives are typically used as part of a detonator's electrical-chemical train leading to the initiation of the main explosive charge, even minor fabrication variances will produce significant variances in the firing times of the individual detonators, destroying the proper operation of adjacent shaped charges due to overlapping pressure waves from the mistimed individual detonators. Therefore, by integrating a compact fast fire set within each detonator, such a distribution system can be eliminated.
Slapper detonator systems (e.g., a chip slapper) can include an energy storage capacitor, a breakdown switch, an exploding foil initiator and a flier. Background information on a fire set/slapper detonator method and system is disclosed in U.S. Pat. No. 5,731,538, titled “METHOD AND SYSTEM FOR MAKING INTEGRATED SOLID-STATE FIRE-SETS AND DETONATORS,” issued Mar. 24, 1998, to O'Brien et al., including the following: “A slapper detonator comprises a solid-state high-voltage capacitor, a low jitter dielectric breakdown switch and trigger circuitry, a detonator transmission line, an exploding foil bridge, and a flier material. All these components are fabricated in a single solid-state device using thin film deposition techniques.” In addition, U.S. Pat. No. 4,788,913, issued to Stroud et al., U.S. Pat. No. 3,978,791, issued to Lemly et al., U.S. Pat. No. 4,471,697, issued to McCormick et al., and U.S. Pat. No. 6,470,802 B1, issued to Neyer et al., disclose “slapper”, foil initiator detonators or multilayer chip slappers.
Background information on an electrical firing system that includes an exploding foil initiator is disclosed in U.S. Pat. No. 6,386,108, titled “INITIATION OF EXPLOSIVE DEVICES,” issued May 14, 2002, to Brooks et al., including the following: “A perforating gun or other downhole tool includes one or more explosive devices that are activatable by corresponding one or more initiator devices, such as capacitor discharge units (CDUs). Each CDU includes an explosive foil initiator (EFI) or some other type of a high-energy bridge-type initiator, an energy source (e.g., a slapper capacitor), and a switch coupling the energy source and the EFI or other bridge-type initiator. An electrical cable is coupled to the CDUs for providing a voltage to energize the energy source in the CDUs to provide energy to each EFI. In response to activation of a trigger signal down the electrical cable, the switch is closed to couple the energy source to the EFI.”
Further background information on an electrical firing system that includes an exploding foil initiator is disclosed in U.S. Pat. No. 5,347,929, titled “FIRING SYSTEM FOR A PERFORATING GUN INCLUDING AN EXPLODING FOIL INITIATOR AND AN OUTER HOUSING FOR CONDUCTING WIRELINE CURRENT AND EFI CURRENT,” issued Sep. 20, 1994, to Lerche et al., including the following: “A fire set circuit provides a discharge pulse to the firing head, and a wireline conductor cable provides a wireline current to the fire set circuit. The firing head includes an outer pressure bulkhead housing adapted for conducting the wireline current from the wireline conductor cable to the fireset circuit, and an exploding foil initiator (EFI) responsive to the discharge pulse from the fire set circuit for initiating the detonation of a secondary explosive.”
Accordingly, there is a need to provide an explosive apparatus that can even more precisely trigger large arrays of fast detonators. The present invention is directed to such a need.